I can't think of any, but perhaps others could jump in and jog my memory. There are preferences, but no hard, fast rules I can think of. For example; if a member nominates a book that isn't available in an eBook format, or one that is only available to a few of the lucky members who live in particular regions, that book has a very slim chance of becoming fully nominated, and an even slimmer chance of winning that month's vote.